This invention relates to the art of conditioning indoor living and working and other enclosed public spaces. More particularly, this invention relates to a system in which the carbon dioxide (CO2) level is monitored and controlled by apparatus in which the CO2 sensor and support circuitry is integral with a thermostat which also serves to conventionally control the temperature range within the conditioned space.
The ever increasing cost of energy has resulted, in recent years, in the construction of homes and buildings which are very well insulated and, further, which carefully provide for keeping the air within the home or building well isolated from the encroachment of outdoor air. This latter feature serves to limit the adverse effect of outdoor air on the desired temperature intended to be maintained within the building occasioned by direct mixing with the indoor air, an effect which is in addition to and independent of the limitation of adverse conduction effects afforded by insulating the conditioned spaces.
However, it has been observed that these highly-efficient, substantially closed, environmental conditioning systems have a serious drawback which can even reach dangerous levels. Because of the natural breathing processes of the occupants of a closed conditioned space and, in some instances, because of the effects-of combustion or similar processes, the concentration level of oxygen (O2) decreases while the concentration of carbon dioxide (CO2) increases. It has been found that when the concentration of CO2 in a conditioned space reaches on the order of 1000 parts per million, breathing becomes noticeably more labored and difficult, particularly for those with breathing problems. Higher concentrations exacerbate these problems and can reach dangerous levels.
Accordingly, recommended maximum allowable concentrations of CO2 for living and working spaces have been formulated and promulgated, and, in some instances, various government agencies have imposed requirements that specify the maximum concentration of CO2 which will be allowed in public conditioned spaces.
Commercial CO2 detectors have been both bulky and have required fairly sophisticated support circuitry because the voltages (or other electrical parameter) generated by or measured across the conventional CO2 sensors, which vary in accordance with changes in CO2 concentration, do so only across a very narrow, low level range. One commonly used CO2 sensor in CO2 detectors constitutes an IR light source directed at an IR light sensor such that the CO2 concentration between the source and sensor subtly affects the response of the source. As a result, the support circuitry has to be able to interpret a suitably compensated signal (for example, for temperature and/or humidity variations) which may change only be a few microvolts across the entire measurement range. It will be apparent that such a CO2 sensor and its support circuitry cannot be integrated with a temperature sensing and control system in a single, compact environmental control unit housing. It will also be apparent to those skilled in the art that such a CO2 sensor cannot be integrated with substantially all the circuitry and components of both the CO2 detection subsystem and the temperature sensing and control subsystem into a single integrated circuit.
It is to overcoming these drawbacks of prior art space conditioning control systems which incorporate a CO2 concentration control feature that the present invention is directed.
It is therefore a broad object of this invention to provide an improved CO2 detector which is sufficiently compact and simple as to be suitable for integration with a thermostat in a single housing.
It is a more specific object of this invention, in a presently preferred embodiment, to incorporate a small, heated, electro-chemical CO2 sensor in such an improved CO2 detector.
Briefly, these and other objects of the invention are achieved in a system for monitoring and modifying the quality and temperature of air within a conditioned space which includes a blower unit, a damper unit for selectively admitting outside air into the conditioned space, a temperature moderating unit and a control unit. The control unit includes a thermostat including a conventional, manually operable, temperature set point adjustment and temperature control apparatus for selectively activating the temperature moderating unit when the temperature of the conditioned space deviates by a predetermined amount from the established temperature set point. The control unit further incorporates integrated CO2 concentration measuring and control apparatus for the conditioned space which includes a small CO2 sensor. In a presently preferred embodiment, the CO2 sensor consists of a cathode disposed on a substrate, an anode disposed on the substrate spaced from the cathode and a solid electrolyte disposed on the substrate intermediate and electrically in contact with each of the cathode electrode and the anode to effect a primary electrical cell. A heater and a heater thermostat are also disposed on the substrate and are connected to a source of electrical energy to maintain the primary electrical cell within a predetermined temperature range on the order of 250xc2x0 C. The metals from which the cathode and anode are fabricated and the chemical composition of the electrolyte being further selected such that the voltage established between the cathode and the anode varies in accordance with the CO2 concentration at the primary electrical cell when the temperature of the cell is within the predetermined temperature range. CO2 concentration modifying apparatus is responsive 1) to the measurement of at least a first voltage level between the cathode and the anode, representing a first predetermined CO2 concentration, for turning on the blower unit; and 2) to the measurement of at least a second voltage level between the cathode and the anode representing a second predetermined CO2 concentration, higher than the first predetermined CO2 concentration, for actuating the damper unit to admit outside air into the conditioned space.
In more simple, alternative configurations, either a blower unit only or a damper unit only may be the elements employed to control the CO2 concentration in the conditioned space. In addition, the use of CO2 concentration sensors (e.g., semiconductor CO2 concentration sensors), suitable for integration with the thermostat, are contemplated in place of the electro-chemical cell.